This invention relates to circulating fluid bed steam generators, and more specifically, to a method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators.
It has long been known in the prior art to employ vertical fuel and air staging in fluid bed units. By way of exemplification and not limitation in this regard, reference may be had to U.S. Pat. No. 4,165,717 entitled "Process For Burning Carbonaceous Materials," which issued on Aug. 28, 1979. In accordance with the teachings of U.S. Pat. No. 4,165,717, carbonaceous material is introduced into a fluid bed in an upright reactor. This carbonaceous material is fluidized in the fluid bed with a primary fluidizing gas introduced at the bottom of the fluid bed. A secondary gas is introduced into the fluid bed at a level above that at which the primary gas is introduced and above the bottom of the fluid bed. Thus, combustion is carried out in the presence of oxygen-containing gases, which are supplied in two partial streams at different height levels of the upright fluid bed, and at least one of the partial streams is used as a combustion-promoting secondary gas and is fed into the combustion chamber on one plane or a plurality of superposed planes. As such, because all oxygen-containing gases required for the combustion are divided into at least two partial streams, which are supplied on different levels, the combustion is effected in two stages. Further, because of the substochiometric combustion in a first lower zone and an afterburning in a second higher zone, there results a "soft" combustion, which eliminates local overheating so that formation of crusts or clogging is avoided and the formation of nitrogen oxide is limited to values below 100 ppm.
As suggested by the preceding, the formation of NO.sub.x can be minimized by vertically staging the mixing of fuel and air. This is done in an effort to ensure that nitrogen in the fuel is not oxidized to form NO.sub.x. The effect of such staging is that there is a staging within the circulating fluid bed steam generator of the combustion that takes place therewithin. In accordance with such staging of the combustion within the circulating fluid bed steam generator, a portion of the fuel is partially burned in the lower furnace of the circulating fluid bed steam generator. Also, for purposes of oxidizing the remaining fuel and the resulting gases generated during combustion, the circulating fluid bed steam generator is provided with overfire air. This overfire air is provided above the location whereat the circulating fluid bed steam generator is provided with fuel.
Thus, by way of summary the conventional manner of staging combustion in a circulating fluid bed steam generator is to feed primary air and/or lower secondary air below the chutes, which commonly are utilized for the purpose of feeding fuel into the circulating fluid bed steam generator. This primary air and/or lower secondary air is fed into the circulating fluid bed steam generator in order to effectuate therewith the partial burning of the fuel in a reducing zone to form N.sub.2 from the nitrogen in the fuel. Overfire or upper secondary air is fed to the circulating fluid bed steam generator above the fuel chutes in order to combust the remaining fuel and reducing gases to achieve low carbon losses, low CO emissions and fully oxidized SO.sub.2 so as to achieve optimal sulfur capture by the sorbent, which for this purpose in accordance with conventional practice is introduced into the circulating fluid bed steam generator.
In accordance with the foregoing, all fuel/air staging is done in the vertical direction. The major difficulty with this is that it presumes that there is good mixing of the fuel and air along the horizontal plane of the circulating fluid bed steam generator. However, it has been found that in fact fuel and air are not well mixed along the horizontal plane of the circulating fluid bed steam generator. Namely, because the fuel and air are not well mixed along the horizontal plane of the circulating fluid bed steam generator, it has been found that some very reducing zones and some air-rich zones occur along the same horizontal plane at the same elevation of the circulating fluid bed steam generator.
Heretofore, in order to extend, beyond that attainable through vertical staging of the mixing of the fuel and air within the circulating fluid bed steam generator, the extent to which NO.sub.x emissions from a circulating fluid bed steam generator are reduced, the practice commonly followed by those in the industry has been to provide the circulating fluid bed steam generator with additional means operative to remove NO.sub.x subsequent to its formation within the circulating fluid bed steam generator. The prior art includes a number of different approaches that have been proposed for use for purposes of reducing NO.sub.x emissions or N.sub.2 O emissions from a fluid bed unit. By way of exemplification and not limitation, one such prior art approach for reducing NO.sub.x emissions from a fluid bed unit is to be found set forth in U.S. Pat. No. 4,880,378 entitled "Combustion Plant With A Device For Reducing Nitrogen Oxides In Flue Gases," which issued on Nov. 14, 1989. In accordance with the teachings of U.S. Pat. No. 4,880,378, a fluid bed unit is provided with means for reducing nitrogen oxides in flue gases, the flue gases being generated as a consequence of the combustion of fuel and air within the fluid bed unit. This means with which the fluid bed unit is provided includes an injection device for injecting into the fluid bed unit a gaseous reducing agent comprising ammonia, and a catalyst arrangement, wherein the catalyst thereof contains elements of the iron group subjectible to a flue gas temperature in excess of 600 degrees C., disposed downstream of the injection device in the direction of flow of the flue gases.
By way of exemplification and not limitation, another such prior art approach for reducing NO.sub.x emissions from a fluid bed unit is to be found set forth in U.S. Pat. No. 5,382,418 entitled "Process For Removing Pollutants From Combustion Exhaust Gases," which issued on Jan. 17, 1995. More specifically, there is disclosed in U.S. Pat. No. 5,382,418 a process for removing the NO.sub.x from a flue gas, the flue gas being generated as a consequence of the combustion of coal, gas or fuel oil. In accordance with this process as set forth in U.S. Pat. No. 5,382,418, an absorbent containing NH.sub.3 and a granular denitrating catalyst is admixed with a flue gas. This absorbent containing flue gas is then introduced into a fluid bed where the flue gas reacts with the absorbent to remove the NO.sub.x therefrom.
By way of exemplification and not limitation, yet another such approach for reducing NO.sub.x emissions from a fluid bed unit is to be found set forth in U.S. Pat. No. 5,178,101 entitled "Low NO.sub.x Combustion Process And System," which issued on Jan. 12, 1993. In accordance with the teachings of U.S. Pat. No. 5,178,101, a process and a system are provided wherein N.sub.2 O emissions, in the course of NO.sub.x emissions being reduced, are simultaneously also reduced. More specifically, in accordance with the teachings of U.S. Pat. No. 5,178,101 the exhaust stream from a fluid bed unit is flowed through a thermal reaction zone in which fuel and air are burned in order to thereby provide a modified heated stream that includes small quantities of combustibles and of oxygen. This modified heated stream is then in turn passed over a catalyst bed under overall reducing conditions, the quantity of oxygen in the stream being in stoichiometric excess of the amount of NO.sub.x and N.sub.2 O, but less than the amount of the combustibles, whereby the NO.sub.x and N.sub.2 O are first oxidized to NO.sub.2 and then the NO.sub.2 is reduced by the excess combustibles.
By way of exemplification and not limitation, yet still another such approach, in this case directed to reducing N.sub.2 O emissions, is to be found set forth in U.S. Pat. No. 5,048,432 entitled "Process And Apparatus For The Thermal Decomposition Of Nitrous Oxide," which issued on Sep. 17, 1991. In accordance with the teachings of U.S. Pat. No. 5,048,432, N.sub.2 O is thermally decomposed by raising the temperature of the N.sub.2 O containing effluent to at least about 1700 degrees F. The N.sub.2 O containing effluent, which is intended to be subjected to the aforesaid treatment, is generated as a consequence of the combustion of fuel within a boiler, e.g., a fluid bed unit. The thermal decomposition of the N.sub.2 O preferably is accomplished by disposing a heating means in the flow path of the effluent from the fluid bed unit. That is, in the case of a fluid bed unit this heating means allegedly for maximum efficiency is advantageously located downstream from the cyclone and upstream from the heat exchangers.
Although the methods, as set forth in the four issued U.S. patents to which reference has been had hereinbefore, for reducing the nitrogen-related emissions from fluid bed units have been demonstrated to be operative for their intended purpose, there has nevertheless been evidenced in the prior art a need for such nitrogen-related emissions reduction methods to be further improved. Namely, there has been evidenced in the prior art a need for a new and improved method for effectuating the reduction of nitrogen-related emissions from a circulating fluid bed steam generator, and, in particular, a new and improved method for effectuating the reduction of NO.sub.x from a circulating fluid bed steam generator. More specifically, a need is being evidenced in the prior art for a new and improved method that, rather than being operative for purposes of effectuating the reduction of NO.sub.x emissions from a circulating fluid bed steam generator by occasioning the removal of the NO.sub.x after the NO.sub.x has been formed therewithin, would be operative for purposes of effectuating the reduction of NO.sub.x emissions from a circulating fluid bed steam generator by enhancing the minimization of the formation of NO.sub.x within the circulating fluid bed steam generator such that since NO.sub.x is not being formed in the circulating fluid bed steam generator the need for the removal thereof is thus obviated.
To this end, there has been evidenced in the prior art a need for such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators that is characterized in a number of respects. One such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render it unnecessary to effectuate the reduction of NO.sub.x emissions from a circulating fluid bed steam generator through the removal of NO.sub.x therefrom since the employment of the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise need to be removed. Another such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render it unnecessary to provide a circulating fluid bed steam generator with selective non-catalytic NO.sub.x reduction equipment for purposes of effectuating therewith the reduction of NO.sub.x therefrom since the employment of the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise need to be removed through the use of such selective non-catalytic NO.sub.x reduction equipment. A third such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render it unnecessary to provide a circulating fluid bed steam generator with selective catalytic NO.sub.x reduction equipment for purposes of effectuating therewith the reduction of NO.sub.x therefrom since the employment of the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise need to be removed through the use of such selective catalytic NO.sub.x reduction equipment. A fourth such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render unnecessary the injection of ammonia into the circulating fluid bed steam generator for purposes of effectuating therewith the reduction of NO.sub.x therefrom since the employment of the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise necessitate such injection of ammonia for its removal. A fifth such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render unnecessary the injection of urea into the circulating fluid bed steam generator for purposes of effectuating therewith the reduction of NO.sub.x therefrom since the employment of the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise necessitate such injection of urea for its removal. A sixth such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render it much less costly to provide and operate a circulating fluid bed steam generator because the employment of the subject new and improved method would render it unnecessary to provide the circulating fluid bed steam generator with additional means to effectuate therewith the reduction of NO.sub.x therefrom since the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise need to be removed through the use of such additional means. A seventh such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would render it much simpler to provide and operate a circulating fluid bed steam generator because the employment of the subject new and improved method would render it unnecessary to provide the circulating fluid bed steam generator with additional means to effectuate therewith the reduction of NO.sub.x therefrom since the subject new and improved method would be operative to prevent the formation within the circulating fluid bed steam generator of NO.sub.x that would otherwise need to be removed through the use of such additional means. An eighth such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would be suitable for application in new circulating fluid bed steam generators. A ninth such characteristic is that such a new and improved method of enhancing the minimization of NO.sub.x formation in circulating fluid bed steam generators would be suitable to be retrofitted for application in existing circulating fluid bed steam generators.
It is, therefore, an object of the present invention to provide a new and improved method for effectuating therewith the reduction of NO.sub.x emissions from a circulating fluid bed steam generator.
It is another object of the present invention to provide such a new and improved method for effectuating therewith the reduction of NO.sub.x emissions from a circulating fluid bed steam generator wherein the reduction of NO.sub.x emissions from the circulating fluid bed steam generator is accomplished as a consequence of enhancing the minimization of NO.sub.x formation in the circulating fluid bed steam generator.
It is still another object of the present invention to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator whereby the utilization thereof obviates the necessity of providing the circulating fluid bed steam generator with selective non-catalytic NO.sub.x reduction equipment.
Another object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator whereby the utilization thereof obviates the necessity of providing the circulating fluid bed steam generator with selective catalytic NO.sub.x reduction equipment.
A still another object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator whereby the utilization thereof obviates the necessity of having to inject either ammonia or urea into the circulating fluid bed steam generator in order to thereby effectuate therewith the reduction of NO.sub.x from the circulating fluid bed steam generator.
A further object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator which is not disadvantageously characterized by the fact that the utilization thereof occasions ammonia slip from the circulating fluid bed steam generator since the utilization thereof obviates the necessity to inject into the circulating fluid bed steam generator either ammonia or urea from whence the ammonia slip would originate.
A still further object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator which is not disadvantageously characterized by the fact that the utilization thereof occasions the contamination of the ash thereof with ammonia or urea since the utilization thereof obviates the necessity to inject into the circulating fluid bed steam generator either ammonia or urea from whence the source of the contamination of the ash would originate.
Yet an object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator which renders the circulating fluid bed steam generator much simpler to provide and operate since the utilization thereof obviates the necessity to provide the circulating fluid bed steam generator with any additional means that would otherwise be required in order to effectuate the removal of NO.sub.x from the circulating fluid bed steam generator to the same extent.
Yet a further object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed steam generator which renders the circulating fluid bed steam generator much less costly to provide and operate since the utilization thereof obviates the necessity to provide the circulating fluid bed steam generator with any additional means that would otherwise be required in order to effectuate the removal of NO.sub.x from the circulating fluid bed steam generator to the same extent.
Yet another object of the present invention is to provide such a new and improved method of enhancing the minimization of NO.sub.x formation in a circulating fluid bed generator that is suitable for application in new circulating fluid bed steam generators and is equally suitable to be retrofitted for application in existing circulating fluid bed steam generators.